1. Field of the Invention
This invention pertains in general to the field of anesthetic breathing apparatuses having a volume reflector unit for providing gas from a reflector volume thereof re-entering into a breathing circuit during inspiration.
2. Description of the Prior Art
In inhalational anesthesia, anesthetic reflectors for the reflection of unused, gas born, anesthetic agents in expiration gas back towards a patient.
For instance in U.S. Pat. No. 4,989,597, an exchanger for open separation is disclosed. The exchanger directly interfaces a ventilator to a patient breathing circuit, and comprises a long, convoluted tube having a narrow diameter, yet large total volume. The open separation of the gases is resulting from the long mixing tube, which contains a volume of two to three liters of gas. There is no divider, such as a filter or membrane, such as in a traditional bag in bottle system, between the separated gas columns of the ventilator and the patient circuit.
The large exchanger tube volume is used as a reflector volume which is connected to the patient circuit via a first outlet/inlet port for receiving gas from the patient circuit during expiration, and for providing gas from the reflector volume into the breathing circuit during inspiration. Further, the reflector unit has an opposite inlet/outlet port which connects the reflector volume to an evacuation line via the ventilator during expiration, and to the ventilator as a controlled source of driving gas during inspiration.
This arrangement allows using the reflector volume with a driving gas pillar virtually moving back and forth in the reflector volume. In this manner an adjacent patient gas pillar alternatingly virtually is moving out of the patient circuit into the reflector volume during exhalation, and back into the patient circuit from the reflector volume during inspiration. Compared to a bag in bottle system, ventilation parameters are improved as there is no interfering membrane between the driving gas and the patient circuit.
In more detail, the reflector volume is thus cyclically filled with previously exhaled gas comprising an anesthetic gas (Xenon) is thus returned to the patient circuit for re-use during a subsequent inspiration. The driving gas, usually oxygen or air, is used as a driving gas pillar pushing the patient gas pillar back into the patient circuit during inspiration. Upon the subsequent expiration, the reflector volume is re-filled with expiratory gas and the driving gas pillar is pushed out of the reflector volume towards evacuation.
The patient gas column and the driving gas column may get admixed, e.g. due to turbulent flow. Furthermore, gas leakage may occur, e.g. leakage at a tracheal tube, or other locations in the anesthetic breathing apparatus.
These factors, singly or in combination, may lead to the driving gas column entering the patient circuit during inspiration. This crossing of the reflector driving gas over the volume reflector is called break-through or penetration of driving gas through the reflector unit and is an undesired condition, as the patient may receive less anesthetic agent than needed. This reduces patient safety as desired clinical effects to be obtained by the anesthetic agent may not be achieved.
In the anesthetic breathing apparatus of U.S. Pat. No. 4,989,597, an admixture of the gas columns occurs in the exchanger. An appropriate concentration of anesthetic gas is maintained by delivery of excess anesthetic gas in the breathing circuit as a compensation for losses due to mixing with the ventilator gas column.
This compensation is made to ensure the intended delivery of anesthetic gas, as even a small amount of mixing of the driving gas into the breathing circuit is regarded as undesired. Awakening or awareness of the patient may occur during anesthesia, which is highly undesired. In order to avoid this dire consequence, the excess amount of anesthetic gas is in U.S. Pat. No. 4,989,597 during every single exhalation wasted into the evacuation.
There is a need to avoid such losses of anesthetic gas from the breathing circuit, e.g. due to economical reasons.
The exchanger in the system disclosed in U.S. Pat. No. 4,989,597 is a long, narrow, convoluted tube having a reflector volume of 2-3 liters.
There is also a need to reduce the reflector volume, e.g. due to limited space when wanting to integrate the reflector volume in an anesthesia machine. Another reason for desired reduced reflector volumes is that ventilatory performance of the anesthetic breathing apparatus would be improved. A compressible reflector volume of 2-3 liters constitutes a large compressible volume that causes delays and is a source of inaccurate ventilatory regulation. It should be kept in mind that the tidal volume for an average grown up is approximately 0.5 liters. For children the tidal volumes may be lower than 100 ml. With regard to such comparatively low tidal volumes, a compressible volume of 2-3 liters is disadvantageous for ventilatory regulation.
Moreover, a well defined front of the gas pillars requires a narrow tube or channel for the reflector volume, which consequently increases gas flow resistance, which is undesired. Increased resistance results e.g. in increased work of breathing of the patient during expiration, or leads to incomplete discharging of the patient's lungs during expiration.
However, reducing the reflector volume to a volume corresponding to a tidal volume, or less, raises other issues, as it would e.g. amplify the losses of anesthetic gas. Also, a reduced reflector volume entails narrower limits when leakage occurs, or when a patient requires large tidal volumes. The reflector volume is simply not sufficient for the task.
Hence, it would be advantageous to provide an improved anesthetic breathing apparatus having a reflector unit, allowing for reduced reflector volume, while maintaining or improving patient safety.